1. Field of the Invention
The present invention generally relates to an image pickup device. More particularly, the present invention is concerned with a solid-state image pickup device comprised of photoelectric conversion elements such as semiconductor photodiodes PD and charge transfer paths formed by charge-coupled devices (hereinafter also referred to as the CCD in abbreviation). The invention is also concerned with a method of driving the same.
2. Description of Related Art
As the typical one of the solid-state image pickup devices, a CCD transfer type device is known and finds utilization in electronic cameras, copying machines or copiers, video apparatuses, and other applications.
In an interline-type solid-state image pickup device, a large number of photodiodes are disposed in the vertical and horizontal directions to thereby form a pixel matrix, wherein a vertical charge transfer path (VCCD) (hereinafter also referred to as a vertical CCD in abbreviation) constituted by charge-coupled device is provided adjacent to each of the photodiode columns. Further, a horizontal charge transfer path (HCCD) (hereinafter also referred to as a horizontal CCD in abbreviation) is formed adjacent to terminal ends of the individual vertical charge transfer paths to thereby allow video charge signals to be read out on a row-by-row basis.
In recent years, there arises a great demand for implementation of the solid-state image pickup device in a reduced or miniature size. In this conjunction, it is noted that even when the chip size is decreased from one inch to 2/3 inches, 1/2 inch or 1/3 inch in an effort to meet the demand, the number of the photodiodes will have to remain substantially unchanged because the number of the pixels in the vertical direction (i.e., columnwise direction) of the solid-state image pickup device has to satisfy the requirements imposed in view of the NTSC (National Television System Committee), PAL (Phase Alternation by Line) standard or other standards.
In order to read out the electric charges from all the photodiodes concurrently and independently from one another, at least two electrodes must be provided for one photodiode PD for enabling electric charge to be stored and isolated from one photodiode to another. However, with the provision of the two electrodes, it is impossible to transfer the electric charge. In other words, in order to transfer the electric charges without mixing them, it becomes necessary to provide at least three electrodes for each of the photodiodes.
However, reduction of the chip size will encounter a limitation in respect to the fine pattern processing or fine patterning, making it difficult to form three or more electrodes for each of the photodiodes PD.
In this conjunction, according to the standards NTSC, PAL or the like, interlaced image signals may be used and thus one frame can be obtained by scanning twice every other line on a field-by-field basis. In other words, one frame is constituted by two fields. To this end, there is employed the vertical charge transfer path having two transfer electrodes for each row of the photodiodes.
FIG. 10 shows only schematically a structure of a conventional solid-state image pickup device known heretofore. Referring to the figure, a number of photodiodes designated by A1, A2; and B1, B2 are disposed in a matrix form, wherein a vertical charge transfer path (VCCD) is disposed in the close vicinity to each of the columns of the photodiodes. Disposed on the vertical charge transfer path (VCCD) are a pair of transfer electrodes for each of the photodiodes in such a manner that each photodiode is coupled to one of the electrodes (shown in phantom) of the vertical charge transfer path (VCCD) for electric charge transfer to the latter. The vertical charge transfer paths (VCCD) have respective bottom ends (as viewed in the figure) which are coupled to a horizontal charge transfer path (HCCD). With such structure of the solid-state image pickup device, electric charges corresponding to every column of photodiodes are transferred to the horizontal charge transfer path (HCCD) in parallel by way of the associated vertical charge transfer paths (VCCD), respectively, whereon these electric charges are serially transferred through the horizontal charge transfer path (HCCD) to the left as viewed in the figure.
The photodiodes (PD) are classified into two sets for a field A and a field B, respectively. The photodiodes for the field A as designated by A1, A2 and so forth and the photodiodes for the field B as designated by B1, B2 and so forth are arrayed alternately with each other in the columnwise direction. Upon reading the field A, only the electric charges of the photodiodes A1, A2 and so forth are transferred to the vertical charge transfer paths (VCCD). Thus, there are available four electrodes for each of the electric charges. Accordingly, the electric charge transfer through the vertical charge transfer paths (VCCD) can be realized by adopting a conventional four-phase driving scheme.
The electric charges read out to the vertical charge transfer paths (VCCD) are then transferred to the horizontal charge transfer path (HCCD) in parallel by one row during a horizontal blanking period to be subsequently transferred serially through the horizontal charge transfer path (HCCD) during a horizontal scanning period to be thereby read out for further processing.
By the way, in order to meet the demand for still image pick-up with a high fineness (or high definition), it is desired to read out or extract the electric charges from all the photodiodes concurrently en bloc. This is because the number of the pixels should be as large as possible for generating a picture of high fineness (or definition). In case all the electric charges are read out from all the pixels simultaneously to the vertical CCD, the electric charges will be present underneath every other transfer electrode in the vertical charge transfer path (VCCD).
As a method of reading all the electric charges from the image pickup device concurrently en bloc, there may be mentioned an accordion type electric charge transfer method. (For more particulars, reference should be made to, for example, an article of A. J. P. Theuwissen and C. H. L. Weijtens contained in PHILIPS TECHNICAL REVIEW, Vol. 43, No. 1/2, 1986.)
According to the accordion type electric charge transfer scheme mentioned above, only the electric charges resident at the lowermost electrode row of the vertical charge transfer paths (VCCD) are first transferred to the horizontal charge transfer path (HCCD). Subsequently, from the second rows of electrodes of the vertical charge transfer paths (VCCD) as counted from the bottom, electric charges are transferred to the horizontal charge transfer pa th (HCCD). When the electric charges of the second row from the bottom move downwardly by a distance corresponding to one electrode, there makes appearance a space corresponding to two electrodes between the electric charges of the third row and those of the second row. This means that the electric charges of the third rows are in the state ready for being transferred. In this manner, the electric charge transfer is carried out progressively from the bottom electrode row of the vertical charge transfer paths (VCCD) by converting an electric charge distribution per two electrodes to an electric charge distribution per four electrodes.
However, when the electric charges are read out by increasing twice the length of electric charge distribution, the electric charges near to the uppermost row of the vertical charge transfer paths (VCCD) are forced to be held at same locations for an extended period. By way of example, let's suppose a pixel matrix including vertical charge transfer path arrays having one thousand pixel rows. In that case, it is only after a distribution of electric charges for five hundred rows has been formed that the electric charges of the uppermost row can start to shift in and along the vertical charge transfer paths (VCCD). To say in another way, representing by 1H a single horizontal period, it is only after lapse of 500H that the transfer of the electric charges in the uppermost electrode row can be started.
At this juncture, it is to be mentioned that the electric charge transfer path realized by a semiconductor material is accompanied with a dark current which exhibits a location dependence characteristic. More specifically, an amount of dark current generated during a predetermined period differs from one to another place or location (position) in the electric charge transfer path. Thus, when the electric charge stored in the vertical charge transfer path (VCCD) is retained at a same location without being transferred, there makes appearance a defect which may be referred to as white defect due to the dark current.
Such being the circumstances, the transfer of electric charges in the uppermost electrode row of the vertical charge transfer paths (VCCD) should be started as early as possible in order to prevent or suppress the occurrence of the white defect due to the location-dependency of the dark current.
In addition, the amount of dark current accompanying the transfer of the electric charge depends on a length of time period during which the electric charge of concern is held in the semiconductor. As mentioned previously, the electric charges in the lowermost electrode row of the vertical charge transfer paths (VCCD) are read out during the first horizontal period, while the electric charges in the uppermost row are read out during the 1000-th horizontal period. Such difference in the time duration for which the electric charges are held in the vertical charge transfer paths (VCCD) before being read out brings about gradual variation or difference in the amount of the dark current on an average, incurring dark current difference or gradation which is referred to as the shading. In order to reduce the difference or variation in the dark current, the time taken for reading out all the pixels (i.e., frame period) should be shortened to a possible minimum.